Method for crystallizing semiconductor material without exposing it to air

ABSTRACT

A semiconductor material and a method for forming the same, said semiconductor material having produced by a process comprising melting a noncrystal semiconductor film containing therein carbon, nitrogen, and oxygen each at a concentration of 5×10 19  atoms·cm −3  or lower, preferably 1×10 19  atoms·cm −3  or lower, by irradiating a laser beam or a high intensity light equivalent to a laser beam to said noncrystal semiconductor film, and then recrystallizing the thus molten amorphous silicon film. The present invention provides thin film semiconductors having high mobility at an excellent reproducibility, said semiconductor materials being useful for fabricating thin film semiconductor devices such as thin film transistors improved in device characteristics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor material, e.g.,containing silicon as the major component. More particularly, thepresent invention relates to a thin film transistor improved inproperties and a process for fabricating the same. The semiconductormaterial according to the present invention enables fabrication of thinfilm semiconductor devices such as thin film transistors havingexcellent device characteristics.

2. Description of the Prior Art

Non-crystalline semiconductor materials (the so-called amorphoussemiconductors) and polycrystalline semiconductor materials have beenheretofore used for the fabrication of thin film semiconductor devicessuch as thin film field effect transistors and the like. The term“amorphous materials” as referred herein signifies not only thematerials having a strict structural disordering in the atomic level,but also includes those having a short range ordering for a distance ofabout several nanometers. More concretely, “amorphous materials” includesilicon materials having an electron mobility of 10 cm²/V·s or lower andmaterials having a carrier mobility lowered to 1% or less of theintrinsic carrier mobility of the corresponding semiconductor material.Accordingly, materials consisting of fine crystal aggregates which arecomposed of fine crystals about 10 nm in size, i.e., the materials knownas microcrystals (having a grain diameter of 50 to 500 Å as calculatedaccording to shira equation in Raman shift) or semi-amorphous materials(having lattice distortion therein and a peak in Raman shift at lessthan 521 cm⁻¹, and having a structure comprising amorphous structure andcrystalline structure with undefined boundary), are collectivelyreferred to hereinafter as amorphous materials.

The use of an amorphous semiconductor such as amorphous silicon (a-Si)and amorphous germanium (a-Ge) in the fabrication of a semiconductordevice is advantageous in that the process can be conducted at arelatively low temperature of 400° C. or even lower. Thus, muchattention is paid now to a process using an amorphous material, becausesuch a process is regarded as a promising one for the fabrication ofliquid crystal displays and the like, to which a high temperatureprocess cannot be applied.

However, a pure amorphous semiconductor has an extremely low carriermobility (electron mobility and hole mobility). Thus, pure amorphoussemiconductors are rarely applied as they are, for example, tochannel-forming areas of thin film transistors (TFTs); in general, thepure amorphous semiconductor materials are subjected to the irradiationof a high energy beam such as a laser beam or a light emitted from aXenon lamp, so that they may be once molten to recrystallize and therebymodified into a crystalline semiconductor material having an improvedcarrier mobility. Such a treatment of high energy beam irradiation isreferred hereinafter collectively as “laser annealing”. It should benoted, however, that the high energy beam not necessary be a laser beam,and included in the high intensity beam is, for example, a powerfullight emitted from a flash lamp which has a similar effect on thesemiconductor material as the laser beam irradiation.

Generally, however, the semiconductor materials heretofore obtained bylaser annealing are still low in the carrier mobility as compared withthose of single crystal semiconductor materials. In the case of asilicon film, for example, the highest reported electron mobility is 200cm²/V·s at best, which is a mere one seventh of the electron mobility ofa single silicon, 1350 cm²/V·s. Moreover, the semiconductorcharacteristics (mainly mobility) of the semiconductor material thusobtained by the laser annealing process suffers poor reproducibility andalso scattering of the mobility values over the single film. Thosedisadvantages lead to a low product yield of semiconductor deviceshaving a plurality of elements fabricated on a single plane.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a thin filmsemiconductor material having a high mobility and a process for formingthe same with excellent reproducibility. More specifically, an object ofthe present invention is to provide a process in which the problems ofthe conventional laser annealing process are overcome, and to provide,accordingly, a practically feasible thin film semiconductor materialimproved in characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the center wavenumber ofthe Raman peak (Raman shift; taken in the abscissa) and the electronmobility (the ordinate) of a laser-annealed silicon film. The oxygenconcentration of the film is found to be 2×10²¹ cm⁻³;

FIG. 2 is a graph showing the relation between the center wavenumber ofthe Raman peak (Raman shift; taken in the abscissa) and the electronmobility (the ordinate) of laser-annealed silicon films with varyingoxygen concentration;

FIG. 3 is a graph showing the relation between the ratio of the fullband width at half maximum (FWHM) of the Raman peak for a laser-annealedsilicon film to the FWHM of the Raman peak for a single crystal silicon(FWHM ratio; taken in the abscissa) and the electron mobility (theordinate), for laser-annealed silicon films with varying oxygenconcentration;

FIG. 4 is a graph showing the relation between the peak intensity ratio(Ia/Ic; taken in the abscissa) and the electron mobility (the ordinate)for laser-annealed silicon films with varying oxygen concentration,where Ia represents the Raman peak intensity (of the peak at awavenumber of about 480 cm⁻¹) for the amorphous component of thelaser-annealed silicon film, and Ic represents the Raman peak intensity(at about 521 cm⁻¹) for the single crystal silicon;

FIG. 5 is a graph showing a position-dependent change of FWHM of theRaman peak for channel-forming areas of field effect transistors, where,the abscissa is X/L with L representing the channel length, and theordinate is FWHM; and

FIG. 6 shows a process for fabricating a field effect transistor.

DETAILED DESCRIPTION OF THE INVENTION

Raman spectroscopy is an effective method for evaluating thecrystallinity of a material, and it has been used with the purpose ofquantitatively evaluating the crystallinity of a semiconductor filmfabricated by a laser annealing process. During an extensive study onthe laser annealing process conducted by the present inventors, it hasbeen found that the center wavenumber, as well as the width, the height,etc., of the Raman peak of a laser-annealed semiconductor film isclosely related to the properties of the semiconductor film.

For instance, the Raman peak at 521 cm⁻¹ for a single crystal siliconwas observed on a laser-annealed silicon film to be shifted to a shorterwavenumber (longer wavelength). It has been also found that the centerwavenumber of this Raman peak is strongly correlated with the carriermobility of the silicon film obtained by laser annealing.

Referring to FIG. 1, an example which illustrates the relation above isexplained. FIG. 1 relates the center wavenumber of the Raman peak(abscissa) to the electron mobility (ordinate) of a laser-annealedsilicon film. The electron mobility was obtained by measuring thecapacitance-voltage (C-V) characteristics on a TFT having fabricatedfrom the silicon film. From FIG. 1, it can be read that the electronmobilities for those having a Raman peak center of 515 cm⁻¹ or higherbehave quite differently from the mobilities of those having a Ramanpeak center below 515 cm⁻¹. More specifically, it can be seen that theRaman peak center is more sensitive to the change in electron mobilityin the peak center wavenumber region of 515 cm⁻¹ or shorter; beyond thiswavenumber, in contrast, a little shift of Raman peak to higherwavenumber side signifies a large increase in electron mobility.

This phenomena is clearly an evidence of the presence of two phases.According to the study of the present inventors, the phase observed witha Raman peak at 515 cm⁻¹ or lower is assumed as a phase which hasachieved atomic ordering in the solid phase without undergoing melting,whereas the phase having a Raman peak of 515 cm⁻¹ or higher is assumablya phase having solidified from a liquid phase which has once experiencedmelting by laser annealing.

The center wavenumber of the Raman peak was 521 cm⁻¹ at maximum, and thehighest observed electron mobility was about 200 cm²/V·s.

In the course of the study for improving the mobility, the presentinventors then found that oxygen, nitrogen, and carbon atomsincorporated in the film greatly influence the mobility. In thelaser-annealed silicon films investigated and which yielded the resultsshown in FIG. 1, the nitrogen and oxygen atoms were both incorporated innegligible amounts, however, the number of oxygen atoms present at thecentral portion of the film was about 2×10²¹ atoms·cm⁻³. Then, thenumber of oxygen atoms in the film was decreased to see the influence ofthis decrease on the relation between the Raman peak center and theelectron mobility.

The concentration of elements other than silicon, such as oxygen,nitrogen, and carbon, is referred hereinafter to the concentration ofthose elements at the central portion of the film. The concentration ofthe elements in the central region is taken into account because despitethe extremely high concentration of those elements in the portion nearthe substrate or at the vicinity of the surface, the impurity elementsin those portions are believed to have little influence in the carriermobility which is to be considered in the present invention. In general,within a coating, the portion low in concentration of those foreignelements exists at the central portion of the film, and it is believedthat the central portion of the film plays an important role in asemiconductor device such as a field effect transistor. Accordingly, “aconcentration of a foreign element” referred simply herein signifies theconcentration at the central portion of the film.

The influence of oxygen concentration on the relation between theelectron mobility and the Raman peak center is illustrated in FIG. 2.Obviously, the electron mobility is significantly improved by reducingthe oxygen concentration of the film. The same tendency was observed forboth carbon and nitrogen. As a possible explanation for the resultsobtained above, the present inventors propose two mechanisms as follows.In the case of laser annealing a film containing oxygen at a highconcentration, the portions low in oxygen atoms serve as crystal nucleito effect crystal growth during the melting and crystallization of thefilm; the oxygen atoms incorporated into the film are driven to theperiphery with the growth of the crystal, and are precipitated at thegrain boundaries. Thus, the film as a whole which results from such aprocess is lowered in mobility due to the barriers created at the grainboundaries. Another possible explanation the present inventors proposeis that the oxygen atoms or the areas rich in oxygen (in general, thoseareas are believed to have a higher melting temperature) function ascrystal nuclei to effect crystal growth. Accordingly, the size of theindividual crystals becomes smaller with increasing population of theoxygen atoms. This leads to a lower crystallinity because of thecrystals having grown into smaller size, which in consequence give alower mobility.

At any rate, decreasing the oxygen concentration in the film iseffective for significantly increasing the electron mobility thereof bylaser annealing. For example, an electron mobility as high as 1000cm²/V·s was obtained by controlling the oxygen concentration to 1×10¹⁹atoms·cm⁻³. Similar results were obtained by lowering the concentrationsof nitrogen and carbon atoms as well. Also, a similar tendency wasobserved on hole mobilities.

Furthermore, as in the case of FIG. 1, the curve relating the electronmobility to the Raman peak position shows typically a knickpoint (i.e.,a bent) irrespective of the oxygen concentration. Referring to FIG. 2,the present inventors denoted the region at the right side (the sidehigher in wavenumber) of the broken lines as a melting and crystallizingregion. This is because a film falling in this region is surmised thatit has undergone melting and recrystallization upon laser annealing. Ahigh mobility was obtained on films falling in this region. Thesemiconductor material in accordance with the present inventionpreferably has a Raman shift of 512 cm⁻¹ or more and is preferablyproduced by laser annealing a semiconductor containing therein oxygen,nitrogen and carbon at a concentration of 5×10¹⁹ atoms·cm⁻³ or lessrespectively in the melting and crystallizing region.

Another similar tendency was observed by the present inventors on therelation between the full band width at half maximum (FWHM) and theelectron mobility. The relation is shown in FIG. 3. The abscissa is thefull band width at half maximum ratio (FWHM ratio), which is definedherein as a ratio of the FWHM of a Raman peak obtained on alaser-annealed film to that obtained on a single crystal silicon. Asmaller FWHM ratio closer to unity represents a film having a structuresimilar to that of a single crystal silicon. Referring to FIG. 3, it canbe seen that the FWHM ratio becomes closer to 1 with increasing electronmobility if films of equal oxygen concentration are compared. Also, aswas the case above in the relation between the mobility and the Ramanpeak center, the electron mobility increased with lowering the oxygenconcentration in the film; similar results were again obtained fornitrogen and carbon atoms. That is, a higher electron mobility wasobtained with decreasing concentration of foreign atoms. Similarly, thesame tendency was observed for hole mobilities. In FIG. 3, the region atthe left hand side of the broken lines shown in the FIGURE is againconsidered as the melting and crystallizing region.

Furthermore, the present inventors found that the electron mobility canbe intimately correlated with the peak intensity of a peak assigned tothe amorphous component within the film. FIG. 4 is a graph showing therelation between a ratio which the present inventors define as a peakintensity ratio (Ia/Ic; taken as the abscissa), and the electronmobility for laser-annealed silicon films with varying oxygenconcentration, where Ia represents the Raman peak intensity (of the peakat a wavenumber of about 480 cm⁻¹) for the amorphous component of thelaser-annealed silicon film, and Ic represents the Raman peak intensity(at about 521 cm⁻¹) for the single crystal silicon. The electronmobility increased with decreasing intensity ratio, provided that theoxygen concentration is the same. This signifies that the electronmobility is higher for films containing less amorphous components.Furthermore, the electron mobility increased with decreasing oxygenconcentration of the film. A similar tendency was observed on theindividual effect of nitrogen and carbon concentrations. The sametendency was observed for hole mobility. The present inventors denoteagain this region at the left hand side of the broken lines shown inFIG. 4 as the melting and crystallizing region.

In addition, the present inventors empirically obtained higher carriermobility with increasing intensity of Raman peaks, and the Raman peakintensity was higher for films having lower concentrations of oxygen,nitrogen, and carbon.

Conclusively, from the foregoing description, a higher carrier mobilitycan be obtained by decreasing the amounts of oxygen, nitrogen, andcarbon incorporated into the film. More specifically, the presentinventors have found that, by reducing concentration of each of theforeign atoms to 5×10¹⁹ atoms·cm⁻³ or lower, more preferably to 1×10¹⁹atoms·cm⁻³ or lower, a high electron mobility can be obtained, saidelectron mobility being as high as 1000 cm²/V·s for a silicon film, forexample. The present inventors found that by decreasing concentrationsof the foreign atoms, this high electron mobility can be amelioratedfurther to a value near to the carrier mobility of a single crystalsemiconductor, and also that the reproducibility of this high mobilitycan be improved. In addition, a hole mobility of from 300 to 500 cm²/V·swas obtained by a similar process.

In the present invention, the laser annealing is carried out in anatmosphere under atmospheric pressure or reduced pressure.

A laser beam or a light equivalent to the laser beam is irradiated to anoncrystal semiconductor containing therein carbon, nitrogen and oxygenat a concentration of 5×10¹⁹ atoms·cm⁻³ or less, preferably 1×10¹⁹atoms·cm⁻³ or less, respectively.

In practice, however, it is difficult to control the concentration ofthe foreign atoms to a value as low as 1×10¹⁶ atoms·cm⁻³ and below, evenif the laser annealing were to be carried out under an extremely highvacuum to a film containing the atoms at a very low concentration of1×10¹⁶ atoms·cm⁻³ or lower. This is because the oxygen gas, nitrogengas, water molecules, carbon dioxide gas, etc., which are present in theatmosphere in trace amounts are taken up in the film during the laserannealing. Otherwise, it is presumably due to the presence of a gashaving adsorbed on the surface of the film, which is then trapped intothe film during the laser annealing.

To circumvent the aforementioned difficulties, a particular fabricationprocess is requisite. One of such processes is to first cover thesurface of an amorphous semiconductor containing the foreign atoms ofoxygen, nitrogen, and carbon at an extremely low concentration of 10¹⁵atoms·cm⁻³ or lower with a protective film such as of silicon oxide,silicon nitride, and silicon carbide, and then laser annealing the filmunder a high vacuum at a pressure of 10⁻⁴ Torr or lower. Such a processenables a semiconductor film having very high mobility, with extremelylow concentrations for oxygen, nitrogen, and carbon. For example, asilicon film having an electron mobility of 1000 cm²/V·s was obtainedwith each of the concentrations of carbon, nitrogen, and carbon beingcontrolled to 1×10¹⁵ atoms·cm⁻³ or lower.

The protective films, such as of silicon oxide, silicon nitride, andsilicon carbide, can be favorably deposited on the surface of anamorphous semiconductor film by continuously depositing the amorphousfilm and the protective film in a same chamber. More specifically, apreferred process comprises, for example, depositing an amorphoussemiconductor film in a chamber equipped with a single vacuum apparatus,using a chemical vapor deposition (CVD) process or a sputtering process,and then, in the same chamber, continuously depositing the protectivefilm while maintaining the same previous atmosphere or by onceevacuating the chamber to an extreme vacuum and then controlling theatmosphere to a one pertinent for the film deposition. However, tofurther improve the yield, the reproducibility, and the reliability ofthe products, it is preferred that independent chambers are provided sothat the films are deposited separately therein, and that the amorphousfilm once deposited in a particular chamber is transferred to anotherchamber while maintaining the high vacuum. The selection of a particularfilm deposition process depends on the plant and equipment investment.At any rate, the important points to be assured are to sufficientlyreduce the amount of oxygen, nitrogen, and carbon in the film and toavoid adsorption of gases on the surface of the amorphous semiconductorand at the boundary between said semiconductor and the protective filmprovided thereon. Even if an amorphous semiconductor film of high purityand a silicon nitride protective film thereon were to be formed, thecarrier mobility would be impaired if the amorphous film were to be onceexposed to the atmosphere. In general, such an amorphous film wouldyield a low carrier mobility even after laser annealing, and moreover,the probability to obtain a high mobility would be very low. This isbelieved due to the gas adsorbed on the surface of the amorphoussemiconductor film which later diffuse into the film at the laserannealing process.

The protective film may be made from any material capable oftransmitting a laser beam or a light equivalent to the laser beam, andusable are silicon oxide, silicon nitride, silicon carbide, and amaterial comprising a mixture thereof and represented by the chemicalformula SiN_(x)O_(y)C_(z) (where, 0≦x≦4/3; 0≦y≦2; 0≦z≦1; and0≦3x+2y+4z≦4). Preferably, the thickness of the film is in the range offrom 50 to 1000 nm.

In the foregoing description, it has been shown that a semiconductorfilm having a high carrier mobility can be obtained by reducing theconcentrations of oxygen, nitrogen, and carbon of the amorphoussemiconductor film and during the laser annealing process. The electronmobility and the hole mobility obtained therefrom are mere averagevalues of the channel-forming area in the field effect transistorsfabricated for the measurement purposes; thus, those values fail to givethe individual mobilities at particular minute portions within thechannel-forming area. However, as was described referring to FIGS. 1 to4 according to the present invention, it has been elucidated that thecarrier mobility can be univocally defined from the parameters such asthe wavenumber of the Raman peak center, the FWHM of the Raman peak, andthe intensity of the Raman peak attributed to the amorphous content incontrast to that of the whole Raman peak. Thus, by the informationavailable from Raman spectroscopy, it is possible to semi-quantitativelyobtain the mobility of a minute area to which a direct measurementcannot be applied.

FIG. 5 is a graph showing a position-dependent change of FWHM of theRaman peak for laser-annealed channel-forming areas of field effecttransistors whose electron mobilities were found 22 cm²/V·s, 201cm²/V·s, and 980 cm²/V·s. Referring to FIG. 5, the abscissa is theposition of the channel forming area, X/L, with L being the channellength of 100 μm, and X being the coordinate of the channel formingarea. Thus, X/L=0 corresponds to the boundary between thechannel-forming area and the source area, X/L=1 represents the boundarybetween the channel forming area and the drain area, and X/L=0.5indicates the center of the channel-forming area.

It can be seen from FIG. 5 that the field effect transistor having anelectron mobility of 22 cm²/V·s has a large FWHM with littlefluctuation. The data is in agreement with the previous observations ofthe present authors referring to FIG. 3, i.e., that the crystallinityapproaches to that of a single crystal with decreasing FWHM, and thatthereby the electron mobility is increased with reducing FWHM. Thesedata show, in addition, that the positional variation in FWHM(dependence of FWHM on position) is low and hence a uniformcrystallinity is obtained over the whole film. The film yielded anoxygen concentration of about 8×10²⁰ atoms·cm⁻³, and it is assumed thatit had not undergone melting during laser annealing.

The channel-forming area of a field effect transistor having an electronmobility of 201 cm²/V·s also yielded an oxygen concentration of 8×10²⁰atoms·cm⁻³. As shown in FIG. 5, the FWHM is lowered over the whole area,and is largely dependent on the position. Moreover, some points yieldedFWHM values comparable to, or even lower than those of thechannel-forming area having an electron mobility of 980 cm²/V·s. A lowFWHM is suggestive of a high electron mobility at that point, and that alocalized portion having a high crystallinity equal to that of a singlecrystal silicon is present. From the viewpoint of mass-production ofdevices, such a material having large local fluctuations is notfavorable despite a high mobility.

The channel-forming area of a field effect transistor having an electronmobility of 980 cm²/V·s yielded an oxygen concentration of about 1×10¹⁹atoms·cm⁻³, which was considerably lower than those of two above. FromFIG. 5 it can be seen that the FWHM is generally low and has lesspositional dependence. The graph indicates that the material has a highelectron mobility as a whole, and that it is composed of a materialhaving a crystallinity well comparable to that of a single crystalsilicon. Thus, such a material is suitable for mass production to beused in devices.

To achieve a high carrier mobility, not only the concentration of theforeign atoms should be lowered, but also the conditions for the laserannealing must be optimized. The conditions for the laser annealing varydepending on the operating conditions of the laser (such as whether thelaser is operated in a continuous mode or in a pulsed mode, therepetition cycle, beam intensity, and wavelength), and the coating. Thelasers to be used in the process is selected fromultraviolet(UV)-emitting lasers such as the various types of excimerlasers, and lasers emitting light in the visible and infrared (IR)regions, such as a YAG laser. A suitable laser should be selecteddepending on the thickness of the film to be laser-annealed. In siliconand germanium materials, the laser annealing is effected at relativelyshallow portions on the surface when a UV-emitting laser is used,because the materials have a short absorption length for UV light. Onthe other hand, because those materials have a longer absorption lengthfor a light in the visible and IR regions, such a beam penetrate deepinto the material to effect laser annealing of the inner portions. Thus,it is possible to obtain a laser-annealed portion only at the vicinityof the film surface by properly selecting the thickness of the coatingand the type of the laser. Anyway, a high carrier mobility can beobtained by optimally controlling the laser conditions such aswavelength and intensity, so that melting and recrystallization mayoccur on the film. To sufficiently effect melting of the film, thelaser-irradiated portion should be maintained at a temperature not lowerthan the melting point of the semiconductor under process for a longduration. In the case of melting silicon, for example, it requiresheating to 1400° C. or higher under the atmospheric pressure; ingermanium, it should be heated to at least 1000° C. under theatmospheric pressure to effect the melting. However, in an extremelyshort period of, for example, 10 nsec as practiced in excimer lasers, atemperature as high as 2000° C. or even higher is achievedinstantaneously and spectroscopically observed. Such a high temperaturedoes not always cause melting of the film. Thus, a definition of atemperature in this case does not make sense.

In addition, a further annealing of a laser-annealed semiconductor filmin hydrogen gas atmosphere in the temperature range of from 200 to 600°C. for a duration of from 10 minutes to 6 hours was effective to obtaina high carrier mobility at an improved reproducibility. This ispresumably due to the formation of many dangling bonds at theinteratomic bonding sites within the semiconductor during the laserannealing, which is effected simultaneously with the crystallization.Such dangling bonds then function as barriers to the carriers. If aconsiderable amount of oxygen, nitrogen, and carbon atoms were to beincorporated in the semiconductor, those foreign atoms may enter intothe interstices of such dangling bonds; however, when the concentrationsof oxygen, nitrogen, and carbon are extremely low as in the presentcase, the dangling bonds remain as they are and therefore require afurther annealing in hydrogen atmosphere after the laser annealing.

The present invention is described in further detail below referring tonon-limiting examples.

EXAMPLE 1

A planar TFT was fabricated and the electric characteristics thereofwere evaluated. Referring to FIG. 6, the process for the fabricationthereof is described. First, an amorphous silicon film (an activationlayer of the TFT) was deposited on a quartz substrate 601 at a thicknessof about 100 nm by a conventional RF sputtering process, at a substratetemperature of 150° C. under an atmosphere consisting substantially of100% argon at a pressure of 0.5 Pa. A 99.99% or higher purity argon gaswas used alone, without adding other gases such as hydrogen. Thesputtering was conducted at a power input of 200 W and an RF frequencyof 13.56 MHz. The resulting film was etched into a 100 μm×500 μmrectangle to obtain a desired amorphous silicon film 602.

The film was confirmed by secondary ion mass spectroscopy (SIMS) tocontain each of oxygen, nitrogen, and carbon at a concentration of 10¹⁹atoms·cm⁻³ or less.

The film was then placed in a vacuum vessel controlled to a vacuum of10⁵ Torr for laser annealing. A KrF excimer laser was operated at apulse duration of 10 nsec, an irradiation energy of 200 mJ, and a pulserepetition of 50 shots, to effect laser annealing by irradiating a laserbeam at a wavelength of 248 nm to the film.

A gate dielectric 603 was then deposited by sputtering on the surface ofthe thus obtained laser-annealed silicon film in an oxygen atmosphere,at a thickness of about 100 nm. The film-deposition was conducted at asubstrate temperature of 150° C. and at an RF (13.5 MHz) power input of400 W. The atmosphere was controlled to be substantially oxygen, and noother gas was intentionally added. The oxygen gas was of 99.9% or higherpurity, and the pressure thereof was 0.5 Pa.

Then, a 200 nm thick aluminum film was deposited by a known vacuumdeposition process, and was further subjected to a conventional dryetching process to remove the unnecessary portions to obtain a 100μmwide gate 604. At this point, a photoresist 605 used in the dry etchingprocess was left unremoved on the gate.

Boron ions were then doped to the whole structure other than the gate byion implantation at a dopant concentration of 10¹⁴ cm⁻². In this case,the gate and the photoresist thereon were utilized as masks to avoiddoping of boron ions to the portion under the gate. Thus were obtainedimpurity areas, i.e., a source area 606 and a drain area 607. Theresulting structure is given in FIG. 6(B).

The resulting structure was then placed in a vacuum vessel controlled toa vacuum of 10⁻⁵ Torr for laser annealing. A KrF excimer laser wasoperated at a pulse duration of 10 nsec, an irradiation energy of 100mJ, and a pulse repetition of 50 shots, to effect the film annealing byirradiating a beam at a wavelength of 248 nm. The impurity areas whichwere made amorphous by the ion doping were thus recrystallized.

The laser-annealed structure was thermally annealed further in hydrogenatmosphere. The substrate was placed in a chamber equipped with avacuum-evacuating means. The chamber was first evacuated to a vacuum of10⁻⁶ Torr using a turbo molecular pump, and this state was maintainedfor 30 minutes. Then a 99.99% or higher purity hydrogen gas wasintroduced to the chamber until the pressure was recovered to 100 Torr,to anneal the substrate at 300° C. for 60 minutes. The vessel was onceevacuated to remove the adhered gases, water, and the like from thefilm, because it had been known empirically that a high mobility withfavorable reproducibility cannot be obtained for the films havingthermally annealed with those impurities being adhered thereto.

Finally, a 100 nm thick silicon oxide film provided on the top of thesource and the drain areas was perforated to form thereon aluminumcontacts 608 and 609. Thus was a complete field effect transistorobtained. A channel is located in the activation layer under the gateelectrode between the source and the drain areas in the field effecttransistor and comprises the intrinsic or substantially intrinsicsemiconductor crystallized by the KrF excimer laser.

The measurement of C-V characteristics on this field effect transistoryielded an electron mobility of 980 cm²/V·s for the channel formingarea. The threshold voltage was 4.9 V. The concentrations of oxygen,nitrogen, and carbon of this field effect transistor were each found bySIMS to be 1×10¹⁹ atoms·cm⁻³ or lower.

EXAMPLE 2

A planar TFT was fabricated and the electric characteristics thereofwere evaluated. First, an amorphous silicon film containing phosphorusat a concentration of 3×10¹⁷ atoms·cm⁻³ was deposited on a quartzsubstrate at a thickness of about 100 nm by a conventional RF sputteringprocess. The amorphous silicon film having deposited at this thicknesscan be wholly annealed by a KrF laser beam at a wavelength of 248 nm.The RF sputtering was performed at a substrate temperature of 150° C.under an atmosphere consisting substantially of 100% argon at a pressureof 0.5 Pa. A 99.99% or higher purity argon gas was used alone, withoutadding other gases such as hydrogen. The sputtering was conducted at apower input of 200 W and an RF frequency of 13.56 MHz. The resultingfilm was etched into a 100μm×500μm rectangle to obtain the desiredamorphous silicon film.

The film was confirmed by secondary ion mass spectroscopy (SIMS) tocontain each of oxygen, nitrogen, and carbon at a concentration of 10¹⁹atoms·cm⁻³ or less.

A gate dielectric was then deposited by sputtering on the surface of thethus obtained silicon film in an oxygen atmosphere, at a thickness ofabout 100 nm. The film-deposition was conducted at a substratetemperature of 150° C. and at an RF (13.56 MHz) power input of 400 W.The atmosphere was controlled to be substantially oxygen, and no othergas was intentionally added. The oxygen gas was of 99.9% or higherpurity, and the pressure thereof was 0.5 Pa.

Then, a 200 nm thick aluminum film was deposited by a known vacuumdeposition process, and was further subjected to a conventional dryetching process to remove the unnecessary portions to obtain a gate at awidth of 100μm. At this point, a photoresist used in the dry etchingprocess was left on the gate.

Boron ions were then doped to the whole structure except for the gate byion implantation at a dopant concentration of 10¹⁴ cm⁻². In this case,the gate and the photoresist thereon were utilized as masks to avoiddoping of boron ions to the portion under the gate. Thus were obtainedimpurity areas, i.e., a source area and a drain area, in the siliconfilm.

The resulting structure was then placed in a vacuum vessel controlled toa vacuum of 10⁻⁵ Torr for laser annealing. A KrF excimer laser wasoperated at a pulse duration of 10 nsec, an irradiation energy of 100mJ, and a pulse repetition of 50 shots, to anneal the film byirradiating a beam at a wavelength of 248 nm from the back of thesubstrate. Thus was the amorphous silicon film recrystallized. In thisprocess, the crystallization of the source or the drain area can beeffected simultaneously with the crystallization of the channel formingarea. The instant process is therefore advantageous in that a favorableboundary with a continuous crystallinity can be obtained with lessdefects, as compared with the process of EXAMPLE 1 in which many defectswere found to generate in the boundary between the source or drain areaand the channel forming area.

The laser-annealed structure was thermally annealed further in hydrogenatmosphere. The substrate was placed in a chamber equipped with avacuum-evacuating means. The chamber was first evacuated to a vacuum of10⁻⁶ Torr using a turbo molecular pump, and then heated to 100° C. tomaintain this state for 30 minutes. Then a 99.99% or higher purityhydrogen gas was introduced to the chamber until the pressure wasrecovered to 100 Torr, so that annealing of the substrate may beconducted then at 300° C. for 60 minutes. The vessel was once evacuatedto remove the adhered gases, water, and the like from the film, becauseit had been known empirically that a high mobility with favorablereproducibility cannot be obtained for the films having thermallyannealed with those impurities being adhered thereto.

Finally, a 100 nm thick silicon oxide film provided on the top of thesource and the drain areas was perforated to form thereon aluminumcontacts. Thus was the structure completed into a field effecttransistor.

The measurement of C-V characteristics on this field effect transistoryielded an electron mobility of 990 cm²/V·s for the channel formingarea. The threshold voltage was 3.9 V. The present field effecttransistor yielded an improved (lower) threshold voltage as comparedwith that of the field effect transistor fabricated in EXAMPLE 1. Thisis assumed attributable to the simultaneous laser annealing of theimpurity areas and the channel forming area, which might havecrystallized uniformly at the same time by irradiating the laser beamfrom the back. Furthermore, the drain current ratio at the ON/OFF of thegate voltage was found to be 5×10⁶.

The concentrations of oxygen, nitrogen, and carbon of this field effecttransistor were each found by SIMS to be 1×10¹⁹ atoms·cm⁻³ or lower. TheRaman spectroscopy of the channel forming area resolved a Raman peak ata center wavenumber of 520 cm⁻¹, having a FWHM of 4.5 cm⁻¹. The presenceof a once melted and recrystallized silicon was evidenced by thoseresults.

EXAMPLE 3

A planar TFT was fabricated and the electric characteristics thereofwere evaluated. First, about 100 nm thick amorphous silicon film and a10 nm thick silicon nitride coating thereon were continuously depositedon a quartz substrate having coated with a 10 nm thick silicon nitridecoating by using a film-deposition apparatus having two chambers. Theamorphous silicon film was deposited by a conventional sputteringmethod, and the silicon nitride film was deposited by a glow-dischargeplasma chemical vapor deposition (CVD).

The substrate was set in a first pre-chamber which was heated to 200° C.and evacuated to a pressure of 10⁻⁶ Torr or lower for 1 hour.Separately, an air-tight first chamber, which is constantly controlledto a pressure of 10⁻⁴ Torr or lower except for the case of filmdeposition, was evacuated to 10⁻⁶ Torr. The substrate was transferredfrom the first pre-chamber to the first chamber and set therein, atwhich point the chamber was evacuated to 10⁻⁶ Torr or lower whilemaintaining the substrate and the target to a temperature of 200° C. fora duration of 1 hour. Then, argon gas was introduced into the chamber togenerate an RF plasma to conduct film deposition by sputtering. A99.9999% or higher purity silicon target containing 1 ppm of phosphoruswas used for the target. The film deposition was conducted with thesubstrate temperature being maintained at 150° C., in an atmosphereconsisting substantially of 100% argon gas at a pressure of 5×10⁻² Torr.No gases other than argon, such as hydrogen, was added intentionally.The argon gas used herein was of 99.9999% or higher purity. Thesputtering was operated at an input power of 200 W and an RF frequencyof 13.56 MHz.

Upon completion of the film deposition, the RF discharge was cut off,and while evacuating the first chamber to a vacuum of 10⁻⁶ Torr, anair-tight second pre-chamber, which is provided between the first andsecond chambers and is constantly maintained to a pressure of 10⁻⁵ Torror lower, was vacuum-evacuated to 10⁻⁶ Torr, so that the substrate maybe transferred therein from the first chamber. Then, an air-tight secondchamber, which is always maintained at a pressure of b 10 ⁻⁴ Torr orlower except for the case of carrying out a film deposition, wasevacuated to 10⁻⁶ Torr to set therein the substrate having transferredfrom the second pre-chamber. The substrate was kept at 200° C. in thesecond chamber, while the chamber was evacuated to maintain thesubstrate under a pressure of 10⁻⁶ Torr or lower for 1 hour.

Then, a gas mixture diluted with hydrogen and comprising a 99.9999% orhigher purity ammonia gas and disilane (Si₂H₆) gas at a ratio of 3:2 wasintroduced in the second chamber to control the overall pressure to 10⁻¹Torr. An RF current was applied to the chamber to generate a plasmatherein, so that a silicon nitride film might be deposited on thesubstrate. The power input was 200 W, at a frequency of 13.56 MHz.

After completion of the film deposition, the RF discharge was cut off.While evacuating the second chamber to 10⁻⁶ Torr, a third pre-chamber,which is provided at one side of the second chamber and having a quartzwindow, was vacuum evacuated to 10⁻⁶ Torr, at which point the substratewas transferred from the second chamber to the third pre-chamber. Then,a KrF excimer laser was operated at a pulse duration of 10 nsec, anirradiation energy of 100 mJ, and a pulse repetition of 50 shots, sothat a laser beam at a wavelength of 248 nm was irradiated to the filmto effect the laser annealing. Thus was the amorphous silicon filmcrystallized.

The process described in the foregoing was particularly effective forimproving the product yield. By thus conducting continuously the laserannealing from the point of film deposition without substantiallydisturbing the vacuum state, a high product yield can be achievedirrespective of the presence of a protective film on the amorphous film.Accordingly, the process is effective on the FETs described in EXAMPLES1 and 2 as well, in which no protective films are provided. Assumably,the films are maintained free from adhesion of dusts, etc., fromadsorption of water and gases, and from scratches and other defects.

The process of continuously conducting the film deposition and theannealing thereof as described above, may be carried out in two ways.One is establishing a film-deposition chamber and separately apre-chamber as described in the foregoing, and providing a window in thepre-chamber to effect the laser annealing through this window. The otherprocess is similar to the first, except that the window is provided tothe film-deposition chamber so that the laser annealing may be effectedsubsequently to the film deposition. The latter process, however,requires etching of the window to remove the coating having adheredduring the film-deposition process, since the coating adhesions make thewindow translucent. Accordingly, the former process is favorable fromthe viewpoint of its applicability to mass production and themaintenance costs.

After the laser annealing is finished in the third pre-chamber, a drynitrogen gas is introduced into the third pre-chamber to recover theatmospheric pressure. The substrate was then taken out from the thirdpre-chamber, and the silicon nitride film having deposited thereon wasremoved by a known dry etching process. The resulting silicon film wasetched into a 100 μm×500μm rectangular shape.

The concentrations of oxygen, nitrogen, and carbon of this field effecttransistor were each 1×10¹⁶ atoms·cm⁻³ or lower, which were eachconfirmed by SIMS performed on a separate film fabricated by the sameprocess.

A gate dielectric was then deposited on the surface of the thus obtainedlaser-annealed silicon film in an oxygen atmosphere by sputtering, at athickness of about 100 nm. The film-deposition was conducted at asubstrate temperature of 150° C. and at an RF (13.56 MHz) power input of400 W. A 99.9999% or higher purity silicon oxide was used as the targetfor sputtering. The atmosphere was controlled to be substantiallyoxygen, and no other gas was intentionally added. The oxygen gas was of99.999% or higher purity, and the pressure thereof was 5×10⁻² Torr.

Then, a 200 nm thick aluminum film was deposited by a known vacuumdeposition process, and was further subjected to a conventional dryetching process to remove the unnecessary portions to obtain a gate at awidth of 100μm. At this point, a photoresist used in the dry etchingprocess was left on the gate.

Boron ions were then doped to the whole structure except for the gate byion implantation at a dopant concentration of 10¹⁴ cm⁻². In this case,the gate and the photoresist thereon were utilized as masks to avoiddoping of boron ions to the portion under the gate. Thus were obtainedimpurity areas, i.e., a source area and a drain area, in the siliconfilm.

The resulting structure was then placed in a vacuum vessel controlled toa vacuum of 10⁻⁵ Torr for laser annealing. A KrF excimer laser wasoperated at a pulse duration of 10 nsec, an irradiation energy of 50 mJ,and a pulse repetition of 50 shots, to anneal the film by irradiating abeam at a wavelength of 248 nm from the back of the substrate. Thus, theimpurity areas which were made amorphous by the ion doping wererecrystallized, that is, the amorphous silicon in the impurity areas wasrecrystallized.

The present process is similar to that described in EXAMPLE 1 in thepoint that the laser annealing is conducted in two steps, however, thepresent process should be distinguished from the previous in that thelaser annealing is effected by irradiating the beam from the back of thesubstrate to form a continuous junction between the impurity regions andthe channel forming area. In particular, the first laser annealing isconducted for the purpose of producing a film of high carrier mobilityby effecting melting-recrystallization, whereas the second laserannealing is conducted at a lower laser output to accelerate ordering inthe microscopic level while avoiding melting of the film, and to therebyreduce the resistance in the impurity area. Since the laser output iscontrolled to a lower level, the crystalline area (mainly the channelforming area) having established in the first laser annealing remainunaffected in the second laser annealing. Furthermore, as was describedin EXAMPLE 2, a boundary having continuous crystallinity reduced indefects can be obtained between the source or drain area and the channelforming area.

Laser annealing by irradiation of an ultraviolet laser beam from thesubstrate surface is effective for the surface portion to which theultraviolet laser beam is irradiated, and is not sufficiently effectivefor deeper portion, that is, there is a high possibility that a highmobility cannot be obtained in the deeper portion. Thereby productionyield may be lowered. Laser irradiation from the back of the substrateis not sufficiently effective for obtaining a high mobility in a portionof the activation layer in contact with the gate electrode. Therefore,laser annealing is carried out twice in this embodiment, that is, thefirst laser annealing is carried out by irradiating a laser beam to theamorphous silicon film from the surface side thereof and the secondlaser annealing is carried out by irradiating a laser beam to the filmfrom the back of the substrate, to increase the production yield and toestablish a continuous junction between the channel forming area and theimpurity areas.

The laser-annealed structure was thermally annealed further in hydrogenatmosphere. The substrate was placed in a chamber equipped with avacuum-evacuating means. The chamber was first evacuated to a vacuum of10⁻⁶ Torr using a turbo molecular pump, and then heated to 100° C. tomaintain this state for 30 minutes. Then a 99.99% or higher purityhydrogen gas was introduced to the chamber until the pressure wasrecovered to 100 Torr, so that annealing of the substrate may beconducted then at 300° C. for 60 minutes. The vessel was once evacuatedto remove the adhered gases, water, and the like from the film, becauseit had been known empirically that a high mobility with favorablereproducibility cannot be obtained for the films having thermallyannealed with those impurities being adhered thereto.

Finally, a 100 nm thick silicon oxide film provided on the top of thesource and the drain areas was perforated to form thereon aluminumcontacts. Thus was the structure completed into a field effecttransistor.

One hundred field effect transistors above were fabricated. Themeasurement of C-V characteristics on these field effect transistorsyielded an average electron mobility of 995 cm²/V·s for the channelforming area. The threshold voltage was 4.2 V in average. The draincurrent ratio at the ON/OFF of the gate voltage was found to be 8×10⁶ inaverage. Each of the field effect transistors thus fabricated waschecked whether it had favorable characteristics on electron mobility,threshold voltage, and drain current ratio. The standard values for theelectron mobility, the threshold voltage, and the drain current ratiowere set to 800 cm²/V·s, 5.0 V, and 1×10⁶, respectively. Ninety-one outof 100 field effect transistors were found as being favorable.

The concentrations of oxygen, nitrogen, and carbon of these field effecttransistors evaluated as favorable were each found by SIMS to be 1×10¹⁶atoms·cm⁻³ or lower.

As described in the foregoing, the present invention provides thin filmsemiconductors of high mobility at an excellent reproducibility. Thedescription above was made mainly on the laser annealing of asemiconductor film having deposited on an insulator substrate made ofquartz and the like, however, the present invention is not only limitedthereto and can be applied to single crystal semiconductors such assingle crystal silicon substrate which are used in, for example,monolithic integrated circuits (ICs). Furthermore, in addition to thesilicon film which was described in great detail in the EXAMPLES, thepresent invention can be applied to a germanium film, silicon-germaniumalloy films, or films of various other intrinsic semiconductor materialsas well as compound semiconductor materials. As described hereinbefore,it should be further noted that the term “laser annealing”, which isused as a means of increasing the mobility of an amorphous film, refersinclusively to means in which a high density optical energy is used,such as a flash lamp annealing. Thus, it should be noted that a processusing a high density optical energy for improving the crystallinity of asemiconductor material is within the scope of the present invention.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1. A method for manufacturing a semiconductor device comprising thesteps of: forming a semiconductor film over a substrate, saidsemiconductor film containing carbon at a concentration of 5×10¹⁹atoms/cm³ or less; and melting said semiconductor film by irradiating aCW laser light to crystallize said semiconductor film.
 2. A method formanufacturing a semiconductor device comprising the steps of: forming asemiconductor film over a substrate, said semiconductor film containingoxygen at a concentration of 5×10¹⁹ atoms/cm³ or less; and melting saidsemiconductor film by irradiating a CW laser light to crystallize saidsemiconductor film.
 3. A method for manufacturing a semiconductor devicecomprising the steps of: forming a semiconductor film over a substrate,said semiconductor film containing nitrogen at a concentration of 5×10¹⁹atoms/cm³ or less; and melting said semiconductor film by irradiating aCW laser light to crystallize said semiconductor film.
 4. A method formanufacturing a semiconductor device comprising the steps of: forming asemiconductor film over a substrate, said semiconductor film containingcarbon at a concentration of 5×10¹⁹ atoms/cm³ or less; forming aprotective layer on said semiconductor film; and melting saidsemiconductor film by irradiating a CW laser light though saidprotective film to crystallize said semiconductor film.
 5. A method formanufacturing a semiconductor device comprising the steps of: forming asemiconductor film over a substrate, said semiconductor film containingoxygen at a concentration of 5×10¹⁹ atoms/cm³ or less; forming aprotective layer on said semiconductor film; and melting saidsemiconductor film by irradiating a CW laser light though saidprotective film to crystallize said semiconductor film.
 6. A method formanufacturing a semiconductor device comprising the steps of: forming asemiconductor film over a substrate, said semiconductor film containingnitrogen at a concentration of 5×10¹⁹ atoms/cm³ or less; forming aprotective layer on said semiconductor film; and melting saidsemiconductor film by irradiating a CW laser light though saidprotective film to crystallize said semiconductor film.
 7. A method formanufacturing a semiconductor device comprising the steps of: forming asemiconductor film over a substrate, said semiconductor film containingcarbon at a concentration of 5×10¹⁹ atoms/cm³ or less; melting saidsemiconductor film by irradiating a CW laser light to crystallize saidsemiconductor film; and annealing the crystallized semiconductor film inan atmosphere containing hydrogen.
 8. A method for manufacturing asemiconductor device comprising the steps of: forming a semiconductorfilm over a substrate, said semiconductor film containing oxygen at aconcentration of 5×10′⁹ atoms/cm³ or less; melting said semiconductorfilm by irradiating a CW laser light to crystallize said semiconductorfilm; and annealing the crystallized semiconductor film in an atmospherecontaining hydrogen.
 9. A method for manufacturing a semiconductordevice comprising the steps of: forming a semiconductor film over asubstrate, said semiconductor film containing nitrogen at aconcentration of 5×10¹⁹ atoms/cm³ or less; melting said semiconductorfilm by irradiating a CW laser light to crystallize said semiconductorfilm; and annealing the crystallized semiconductor film in an atmospherecontaining hydrogen.
 10. A method for manufacturing a semiconductordevice comprising the steps of: forming a semiconductor film over asubstrate, said semiconductor film containing carbon at a concentrationof 5×10¹⁹ atoms/cm³ or less; patterning said semiconductor film to forma semiconductor island; melting said semiconductor island by irradiatinga CW laser light to crystallize said semiconductor island; forming agate insulating film over the crystallized semiconductor island; andforming a gate electrode over said gate insulating film.
 11. A methodfor manufacturing a semiconductor device comprising the steps of:forming a semiconductor film over a substrate, said semiconductor filmcontaining oxygen at a concentration of 5×10¹⁹ atoms/cm³ or less;patterning said semiconductor film to form a semiconductor island;melting said semiconductor island by irradiating a CW laser light tocrystallize said semiconductor island; forming a gate insulating filmover the crystallized semiconductor island; and forming a gate electrodeover said gate insulating film.
 12. A method for manufacturing asemiconductor device comprising the steps of: forming a semiconductorfilm over a substrate, said semiconductor film containing nitrogen at aconcentration of 5×10¹⁹ atoms/cm³ or less; patterning said semiconductorfilm to form a semiconductor island; melting said semiconductor islandby irradiating a CW laser light to crystallize said semiconductorisland; forming a gate insulating film over the crystallizedsemiconductor island; and forming a gate electrode over said gateinsulating film.
 13. A method for manufacturing a semiconductor devicecomprising the steps of: forming a semiconductor film over a substrate,said semiconductor film containing carbon at a concentration of 5×10¹⁹atoms/cm³ or less; and melting said semiconductor film by irradiating aCW laser light to crystallize said semiconductor film in vacuum.
 14. Amethod for manufacturing a semiconductor device comprising the steps of:forming a semiconductor film over a substrate, said semiconductor filmcontaining oxygen at a concentration of 5×10¹⁹ atoms/cm³ or less; andmelting said semiconductor film by irradiating a CW laser light tocrystallize said semiconductor film in vacuum.
 15. A method formanufacturing a semiconductor device comprising the steps of: forming asemiconductor film over a substrate, said semiconductor film containingnitrogen at a concentration of 5×10¹⁹ atoms/cm³ or less; and meltingsaid semiconductor film by irradiating a CW laser light to crystallizesaid semiconductor film in vacuum.
 16. A method according to any one ofclaims 1 to 15, wherein said CW laser light is a YAG laser light.
 17. Amethod according to any one of claims 1 to 15, wherein saidsemiconductor film comprises silicon or silicon-germanium alloy.
 18. Amethod according to any one of claims 4 to 6, wherein said protectivelayer comprises a material expressed by a formula SiN_(x)O_(y)C_(z),where 0≦x≦4/3, 0≦y≦2, 0≦z≦1, and 0<3x+2y+4Z≦4.